1. Field of the Invention
The invention is an assay device useful in determining the presence and concentration of uric acid in a liquid sample, such as urine.
2. History of the Invention
Dry phase analyte assay test strips (also known as xe2x80x9cone-stepxe2x80x9d assays, requiring only addition of sample to the test strip) to detect the presence or concentration of analytes such as glucose and hormones in body fluids are widely used in clinical laboratories, physician""s offices, hospitals, and homes. Important advantages of such test strips include low cost, ease of use, relatively accurate assays and a short response time. However, the manufacture of test strips for one-step is detection of uric acid has been hampered by thermal instability of the uricase enzyme reagent, whose activity can be quickly impaired under common conditions of manufacturing, storage and use (especially with fluids samples of variable pH, such as urine). Thus, uric acid is typically measured in blood or serum using multiple step liquid sample assay protocols.
The invention consists of a one-step test strip and methods for its use in detecting the presence and concentration of uric acid in liquid samples, including urine. The test strips include a patch impregnated with a working solution, including stabilized uricase. The test strips of the invention are stable in that they can be stored for relatively long periods of time without loss of uricase activity.
In particular, the test strips consist of a porous material impregnated with an enzyme-stabilized, pH-controlled, buffered working solution in which uricase is stabilized against thermal degradation The unicase reagent is further protected against degradation during manufacture of the test strips by use of a process consisting of impregnation of the strip with the working solution and drying to remove solvent. Application of the working solution may be followed by a second impregnation of the test strip with a concentrated buffer. Thus, if the buffer in the working solution has a salt concentration (using phosphate salt as an example) of about 1M and a pH of 8.5 to 9.5, no second application of buffer is required. However, if a working solution having a buffer with a salt concentration of 50 mM to 2M and a pH between 6.5 and 7.0 is utilized, impregnation of the working solution is followed by drying of the test strip and application of more concentrated buffer with a similar pH.
For use in detection of uric acid, a visually detectable signal indicative of the presence and/or concentration of uric acid present in an analyte sample is incorporated into the test strip. Conveniently, the visually detectable signal is produced by a color-forming entity, such as a chromogen.
Kits incorporating the test strips of the invention are also provided. Components of the kits may include test strips, color charts for comparison to color observed after assay performance using the test strips and storage containers for the test strips after use in an assay (for subsequent reference).
Other features and advantages of the invention, eg., stable test strips useful for determining uric acid levels, will be apparent from the following detailed description, the drawings, the Examples and from the claims.